Apparatus for fabricating coating and method of fabricating the coating

ABSTRACT

In an apparatus for fabricating a carbon coating, an object such as a magnetic recording medium is disposed on a side of an electrode connected to a high-frequency power supply. Ultrasonic vibrations are supplied to the object. Discharge is generated between the electrode connected to the high-frequency power supply and a grounded electrode to fabricate a carbon coating on the surface of the object. Also, an electrode interval is set to 6 mm or less, pressure between the electrodes is set to 15 Torr to 100 Torr, whereby high-density plasma is generated to form an ion sheath on an anode side. Therefore, a coating is fabricated on the surface of the object by bombardment of ions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for fabricating a hardcarbon coating to be prepared as a surface protective film for amagnetic tape, an optical magnetic disc or the like, and a method ofmanufacturing such a hard carbon coating. Further, the present inventionrelates to an apparatus for manufacturing a magnetic disc medium havinghigh durability and high recording density and excellent in productivityon a polymeric substrate material, and more particularly to an apparatusfor manufacturing a protective film having functions of wear resistanceand lubricating ability, which is industrially applied to an imageequipment, an information equipment field and the like.

2. Description of the Related Art

Hitherto, there has been known a technique by which a diamond-shapedcarbon coating is formed. The diamond-shaped carbon coating has adiamond structure and is also called a diamond-like carbon (DLC) film.Hereinafter, this carbon coating is referred to as “hard carboncoating”.

The hard carbon coating is coated on a surface of resin or a polymericfilm and can be used as a wear-resistant layer or a protective layer.When manufacturing the hard carbon coating, a film forming apparatussuch as a plasma CVD apparatus shown in FIG. 2 is used. In the filmforming apparatus shown in FIG. 2, a pair of electrodes 112 and 114 aredisposed within a vacuum vessel 111. One electrode 112 is connected to ahigh-frequency power source 115 (generally, 13.56 MHz) whereas the otherelectrode 114 is grounded. An object or a substrate 113 on which a filmis to be formed is disposed on the electrode 112 side to which ahigh-frequency power (voltage) is supplied. Also, although not shown, asupply system, an exhaust system for a reactive gas, and a matchingdevice for supplying voltage are provided.

In the plasma CVD apparatus shown in FIG. 2, electrons are charged ontothe substrate 113 and the electrode 112 (electrode opposed to thegrounded electrode 114) connected to the high-frequency power source115. Therefore, H+-ions and H-radicals contributing to heightenedquality of the film by action of self-bias collide with the object 113,thereby to prepare the carbon coating having the diamond structure.

The hard carbon coating thus prepared can be used as the protective filmof the magnetic recording medium such as a magnetic tape or an opticalmagnetic disc, etc. Since these magnetic recording media are made ofmagnetic material, it is necessary to protect the media from being mixedwith a foreign matter or being damaged. For example, a DC bias isapplied in addition to high-frequency discharge, whereby a carboncoating in which pin-holes of 10² to 10⁵ per mm² are formed is preparedon a surface of the magnetic recording medium.

However, according to the experiments of the inventors, it has beenascertained that the hard carbon coating having the pin-holes thereinlacks long-term reliability as a protective film because moisture isinfiltrated into the pin-holes. Also, it has been ascertained thatimprovements in hardness and adhesion of the hard carbon coating and theprevention of generation of the pin-holes are not always performedtogether.

Recently, there is a tendency to heighten the density of the magneticrecording medium. As a conventional magnetic recording medium, there hasbeen known a coating type in which γ-Fe₂O₃ powders, CrO powders, pureiron powders or the like which are used for an audio or video tapematerial are coated on a polymeric substrate material together with anabrasive material and a binder. Further, a high performance magneticrecording medium on which a metal magnetic material has beenvapor-deposited is used.

Furthermore, there has been known a technique by which a coating mainlycontaining carbon (also called a carbon film, a DLC or a hard carbonfilm) is formed on a surface of these magnetic recording medium, therebyto obtain a surface protection, a wear resistance or a lubricatingability. In general, the coating mainly containing carbon is fabricatedby the CVD method typical of the plasma CVD method.

In the typical plasma CVD method, a substrate is located on ahigh-frequency voltage supply electrode (cathode) side, and a self-biasformed in the vicinity of the cathode allows a high-hard film to befabricated. In general, a carbon film with a high hardness cannot befabricated on a grounded electrode (anode) side.

When a coating mainly containing carbon is formed using the plasma CVDmethod of the parallel plate type, an organic resin substrateconstituting an objedt of the magnetic recording medium must be locatedon a cathode electrode side. The magnetic recording medium forhigh-density recording is generally obtained by vapor-depositing themetal magnetic material. Therefore, if such an object is in contact withthe cathode electrode, the object constitutes a part of the electrode,and therefore a high-frequency electric field is leaked as a result ofwhich discharge occurs in an undesired region. There is a highpossibility that the organic resin film constituting the object isdamaged by such discharge, causing a problem on the stability andreliability of production.

Furthermore, fabricating the hard carbon film which is a protective filmat the same time as the roll to roll type magnetic layer fabricatingprocess is impossible because the carbon film forming speed is low.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of forming anovel hard carbon coating.

Another object of the invention is to provide a method of fabricating ahard carbon coating which is fine, has high hardness and adhesion andreduces the number of pin-holes formed therein as a protective film of amagnetic recording medium.

Still another object of the invention is to provide an apparatus capableof stably producing a hard carbon film on a surface of a magneticrecording medium including an electrically conductive metal magneticlayer with high reliability, that is, to provide an apparatus capable offabricating a carbon film having a sufficient wear-resistance andlubricating ability under the condition where it is in contact with ananode which constitutes a grounded electrode.

Yet still another object of the invention is to provide an apparatuscapable of forming a film at a high speed to the degree of being capableof fabricating a hard carbon film which is a protective film at the sametime as a process of fabricating a magnetic layer.

Yet still another object of the invention is to provide an apparatuscapable of restraining generation of a flake caused by a stain of anelectrode, by forming a film at a high speed.

In the present invention, the hard carbon coating is fabricated while anobject is subjected to ultrasonic vibrations. In particular, in theplasma CVD apparatus constituted by a parallel plate in which ahigh-frequency power supply is connected to one electrode on a sidewhere the substrate on which the hard carbon coating is to be formed islocated, and the other electrode is grounded, the carbon coating isfabricated under the condition where the ultrasonic vibrations ispropagated to the substrate.

Further, in the present invention, a magnetic recording medium such as aband-shaped film object (for example, a taped film) or the like is usedfor an object, this object travels while it is subjected to ultrasonicvibrations, and the hard carbon coating is fabricated on the surface ofthe object (substrate) as a protective film.

By subjecting the object to ultrasonic vibrations, cluster-shaped carbonof small particles or carbon molecules can be deposited on the surfaceof the object, and the carbon coating formed can be fine and uniform inquality. This is because the object is ultrasonic-vibrated, wherebylarge carbon molecules is repelled from the substrate which is vibratingand molecules having a specified size or less are liable to be depositedon the surface of the object.

Further, a film shaped object is used for an object, and the objectvibrates in a direction of an inter-electrode, thereby to realize astate where bias is applied in a pulse mode or a high-frequency mode.

Still further, the number of pin-holes per 1 mm² in the hard carboncoating is 30 or less. The thickness of the hard carbon coating is 50 to2000 Å, preferably 100 to 500 Å.

If at lease one kind of element selected from Si, B, N, P and F iscontained by 20 atoms % or less in a hard carbon coating used as thisprotective film, adhesion of the coating is improved to provideelectrical conductivity. For example, Si and P are contained in the hardcarbon coating, thereby to obtain a protective film having highconductivity and making it difficult to charge with static electricity.

When using a slender film-shaped object such as a magnetic tape,ultrasonic vibrations are given to the object, thereby being capable ofpreventing the substrate from being contracted. Also, particles (apowder like raw material which does not come to a film) attached to thesurface of the substrate can be removed by ultrasonic vibrations.

Further, ultrasonic vibrations are given to the substrate in thedirection of the inter-electrode arranged in parallel, thereby torealize a state where AC bias voltage is applied between electrodes.This action is particularly effective in the case of using a film-shapedsubstrate such as a magnetic tape which is flexible and has a largeamplitude.

According to the present invention, in an apparatus for fabricating acoating in which a first electrode to which high-frequency electricfield is applied and a grounded second electrode are arranged so as tobe opposed to each other, high-frequency electric field is applied togenerate plasma between the first and second electrodes, and raw gasintroduced in the plasma is activated to fabricate a coating, aninterval between the first and second electrodes is 6 mm or less,pressure between the electrodes is 15 to 100 Torr. That is, when theelectrode interval is 6 mm or less and pressure therebetween is 15 to100 Torr, even though the substrate is in contact with the secondelectrode, the carbon film with high hardness can be fabricated. This isascertained by the experimental view of the inventors.

Prior to the above-mentioned view, the inventors have observed thephysical property of a plasma within a high pressure range (5 Torr to760 Torr) remarkably higher than a pressure range (10 mTorr to 1 Torr)generally selected by the plasma CVD. The reason why attention is paidto the pressure range higher than what is generally expected is that thespeed of film forming by the usual plasma CVD is desirable to improveexcessively.

Considering film forming processes in the plasma CVD (generation of aradical, transport of the radical onto the surface of the substrate, andreaction of the radical on the surface), if (1) a radical densityforming a precursor of a film, and (2) the transport efficiency of theradical onto the surface of the substrate can be improved, then a filmforming speed can be improved. In the case of the plasma CVD, since theradical occurs in the whole plasma space, it can be presumed that thegeneration of the radical influences the film forming speed greater thanthe transport of the radical. The radical density can be increased byelevating a reaction pressure, and it can be expected that the filmformation under high pressure results in a high-speed film formation.

In the film forming process, (3) the reaction of the radical on thesurface of the film (suppression of surface desorption) is consideredother than the above-mentioned cases (1) and (2). However, in the caseof a low-temperature process such as the plasma CVD, the surfacereaction rate is not determined, but the reaction process of the radicalon the surface of the film to the film forming speed does notcontributed thereto. In the case of forming the hard carbon film, theion action on the surface greatly influences the film quality. That is,bombardment of ions positively acts while the hard carbon film isformed, whereby strong coupling in the film remains whereas weakcoupling is cut off. Therefore, in general, the object is disposed onthe cathode side, and a film is formed using self-bias.

Even though the increase in the radical density can be realized bypressure increase when forming a film, there is no sense in the case ofgreatly changing the physical property of plasma which is a premise ofthe radical generation by pressure rise. Accordingly, the inventors haveobserved plasma in high pressure range (5 Torr to 760 Torr), asmentioned above.

First, requirements for generating plasma in the high pressure range (5Torr to 760 Torr) must be set. The reason that low-pressure glowdischarge is conventionally generated in a pressure range of 10 mTorr to1 Torr is because discharge is most liable to occur in this pressurerange, that is, discharge is stabilized. The number of times whenparticles existing between the parallel plate electrodes having anelectrode interval d (in the case of the usual low-pressure glowdischarge, d=several tens mm) collide with electrons (it is assumed thatelectrons are accelerated by electric field between electrodes so as tofly from one electrode to the other electrode) is proportional to itsatmospheric pressure. That is, the number of times is inverseproportional to a mean free stroke. Therefore, when the pressure is lowand the number of times of collision is reduced, because electrons havesufficient energy, ionization of the particles is generated by thatcollision. However, due to low pressure, the particles per se arereduced thereby not forming plasma. On the other hand, when pressure ishigh, the number of collision times of electrons is increased wherebyelectrons cannot have sufficient energy till the succeeding collision,as a result of which the particles cannot be ionized even though theycollide with the electrons. This is known as Paschen's law, anddischarge start voltage V becomes a function of product (pd product) ofpressure p and an electrode interval d, so that a minimum dischargestart voltage Vmin exists when the pd product has a certain value. Thatis, when plasma is to be generated in a high-pressure range, it isrequired that sufficient electric field is given to electrons in orderto ionize particles for a short free stroke.

To cope with this, the electrode interval d is narrowed and voltageapplied between electrodes is heightened. There is a limit in an effectresulting from heightening the voltage applied between the electrodes.That is, in the case of glow discharge, an electric field distributionwithin plasma is not uniform so that an electric field is largest on asheath portion formed in vicinity of the electrodes. Then, an electricfield is developed on a positive column portion connected to the sheathportion. The length of the sheath portion is as long as a Debye lengthinherent to plasma, and the electric field of the positive columnportion occupying most of space is not so large. Therefore, even thoughlarge voltage is applied between electrodes, a substantial increase ofthe electric field on the positive column portion occupying most ofspace cannot be much expected.

Because an increase in voltage between the electrodes is applied to thesheath portion, the ionization in this range is facilitated. When theelectric field on the sheath portion exceeds a predetermined strength,accelerated electrons collides with the surface of the electrode,thereby to cause thermionic emission. A mechanism of dischargingelectronics from the electrode in a glow discharge state is fieldemission and secondary electron emission. However, when the thermionicemission is generated, the electric field consumed with electronemission from the electrode does not almost exist, and the electricfield of that amount occurs on the sheath portion. In this case,electrons on the sheath portion is further accelerated whereby theelectrode is heated. Accordingly, thermorunaway occurs as far aselectrode potential is maintained. Such a state has negative resistance,and when current flows over the whole paths, discharge state shifts toarc discharge.

Consequently, it is effective to narrow the electrode interval in plasmageneration within a high-pressure range. There exists a lower limit ofthe electrode interval. In order to allow plasma to exist, the electrodeinterval at least several times as long as a Debye distance is required.The Debye distance λ is represented by the following expression.λ=(ε₀ ·κ·Te/q ² ·Ne) ^(1/2)where ε₀ is the dielectric constant of vacuum, κ is Boltzmann'sconstant, q is a charge elementary quantity, Te is temperature ofelectrons and Ne is density of electric charge.

Since plasma has the electric charge density of about 10¹⁵/m³ and theelectronic temperature of about 2 eV in an embodiment of the presentinvention, the Debye distance becomes about 0.3 mm. Therefore, theelectrode interval is preferably 1 mm or more.

As mentioned above, although discharge in a pressure range of 1 Torr to760 Torr is possible, the physical property of plasma is remarkablychanged. In the pressure range of about 100 Torr to 760 Torr, asapparent from the above-mentioned shift mechanism to arc discharge, thedischarge is liable to be unstable in the usual electrode structure.

Consequently, a heat-resistant dielectric substance is applied onto thesurface of the electrode so that a negative resistance is not exhibitedin the whole system even though discharge exhibits a negativeresistance. Since this dielectric substance has a positive resistance,the whole system provides a positive resistance. In this case, becausethe dielectric substance is connected in series in the equivalentcircuit, it is necessary to make an a.c. electric field develop betweenthe electrodes.

Further, in this range, pressure is high and possibility of collisionand re-coupling of ions and electrons in a space is high, whereby plasmais liable to disappear. Therefore, it is necessary to facilitatediffusion of ions and electrons (in particular, diffusion of ions) so asto broaden a plasma region. For that purpose, it is effective to addrare gas having a matasable state, in particular, helium or argon. It ispreferable that rare gas is 80% or more of the total gas.

Also, it is preferable to diffuse particles constituting plasma by theaid of a magnetic field. In particular, it is preferable to provide thedistribution of a magnetic field so that a magnetic flux diverges fromthe center of the electrode toward the exterior. If such a magneticfield distribution can be obtained, then electrons drift along thediverged magnetic flux, and positive ions are attracted so that anelectric field generated by the electrons is canceled, as a result ofwhich plasma is diffused.

As mentioned above, within the pressure range of about 100 Torr to 760Torr, it is necessary to insert a dielectric substance onto the surfaceof the electrode and to add rare gas. However, in the pressure of about100 Torr or less, the dielectric substance and rare gas are not alwaysnecessary. However, the existence of the dielectric substance and raregas under the pressure of about 100 Torr or less has such an effect thatdischarge is stabilized. However, there arises problems that the costsare increased and the film forming speed is lowered.

The inventors have observed the physical property of plasma under thepressure of 5 Torr to 750 Torr. Gas used in the experiment is argon, andelectrodes between which a dielectric substance is inserted forstabilization of plasma is used. The dielectric substance is formed of asintered alumina having a thickness of 0.5 mm, and a frequency is 13.56MHz.

As a typical physical value of plasma, a sustaining voltage necessaryfor maintaining plasma, the electron temperature (Te), and the electrondensity (Ne) is measured. The electron temperature and the electrondensity are measured by using the Langmuir probe method (single probemethod), and the sustaining voltage is measured by a terminal voltage ofa power source. FIGS. 12 and 14 show measured results.

FIG. 14 shows the electron temperature (Te) and the electron density(Ne). Regarding the electron density, when the probe voltage is appliedin a positive voltage direction, an electron saturated current regioncan be observed. However, there exists a pressure range (60 Torr ormore) which cannot be observed in this region. Therefore, since theelectron density cannot be calculated in the pressure range, theelectron density in 60 Torr or more is not shown. The electron densityin 40 Torr or less gradually rises from 1×10¹⁴/m³ to 1.7×10¹⁴/m³ aspressure rises, and rapidly rises to 8×10¹⁴/m³ in a range of 40 Torr to60 Torr. This shows the fact that arc discharge is locally generatedwith the boundary of pressure of about 40 Torr, and also shows thatplasma in a range of 40 Torr to 60 Torr becomes unstable. However, byusing this, a plasma with a considerably high density can be obtained.

FIG. 12 shows the electron temperature (Te) and the sustaining voltage.The sustaining voltage necessary for maintaining plasma is a valuerepresentative of facility in dealing with plasma, and is desirably aslow as possible. In view of this fact, it is preferable that a minimumvalue is exhibited in a pressure of 10 Torr to 100 Torr and plasma isused in this pressure range. On the other hand, the characteristic curveof the electron temperature has a minimum value in pressure of 60 Torrand a U-shaped form. The electron temperature in an intermediatepressure range of 15 to 100 Torr is lower than that in a lower pressureand a higher pressure than 10 to 100 Torr, that is, 3 eV or less.

The above-mentioned results are typical and therefore do not exhibit allthe results. When the used gas is altered to helium, neon or the like,hydro-carbon gas is added to the used gas, or gas flow rate is changed,then the results are different. For example, the minimum pressure of theelectron temperature is changed in a range of 60 Torr to 100 Torr,pressure in which the electron density rapidly increases is changed in arange of 40 to 80 Torr, and pressure in which the sustaining voltage isminimum is changed in a range of 20 Torr to 100 Torr. However,substantially the same results are qualitatively obtained.

As a result of the above-description, it is preferable to lower thesustaining voltage in the intermediate pressure (15 Torr to 100 Torr) inview of facility of using the device, lighting of a power supply andlowering of the costs, and it is preferable to increase electron densityin view of an increase in radical density. Furthermore, in the range ofthe intermediate pressure, because the electron temperature is lowered,it is disadvantageous to generation of radical. However, because plasmapotential elevates with respect to an anode which is a groundedpotential, bombardment of ions to the anode occurs. This is veryconvenient to fabricate a hard carbon film located on the anode side.The reason will be described below.

Regarding electrons and ions within plasma, electrons readily move underan electric field having the identical strength because of a differencein mass between electrons and ions, compared with ions. Accordingly, thepossibility that electrons reach a vessel becomes higher than that ofions. If the vessel is insulative, then the vessel is negativelycharged. If the vessel is conductive, assuming that the vesselcontacting with plasma is at the same potential as plasma, current flowsin a direction of plasma through the vessel Since flowing of current iscontrary to the condition of charge neutrality, plasma potential ischanged in a positive direction with respect to the vessel so thatflowing of current is canceled. That is, regardless of the vessel beingmade of a conductive substance or an insulative substance, plasma ispositively charged with respect to the vessel according to a differenceof mobility between electrons and ions.

This exhibits that an ion sheath exists on the grounded electrode side.It is certain that the ion sheath also exists on the cathode (voltagesupply electrode side). However, usually, because an ion sheath which isnaturally generated is sufficiently smaller than the sheath generated byself-bias, it is ignored. An electric field generated by the ion sheathcan deal with the ion sheath as an equivalent of a capacitor having anelectrical double layer.

Assuming that the speed of electrons is Boltzmann-distributed, theelectron density within the ion sheath is reducedexponential-functionally so that a space charge within the ion sheathprovides an exponential functional curve. It is appropriate that theboundary between the ion sheath and plasma is defined by a positionwhich has a potential of the degree of Vt=−κ·Te/2q with respect to bulkpotential of plasma. This results from moving the electrons withinplasma bulk with energy of the degree of κ·Te/2q.

As the electron temperature is heightened, a thickness d of the ionsheath is decreased because electrons is infiltrated into the ionsheath, thereby to increase a capacitance C of the electrical doublelayer. Inversely, as the electron temperature is lowered, thecapacitance C of the electrical double layer is reduced.

The charge quantity stored in the ion sheath is proportional to theelectron density (Ne), that is, the ion density (Ni), and thereforevoltage V developed between both ends of the electrical double layer isrepresented by the following expression.V=Q/C=(Ne)^(2/3)·d/ε₀·S

-   -   where d is the thickness of the ion sheath and S is an area of        the electrode. That is, as the electron temperature is lowered,        the electric field within the ion sheath is more strengthened,        and bombardment of ions to the anode is enlarged.

Hitherto, a hard carbon film could not be fabricated on the anode side.On the other hand, in the device of the present invention, a pressurerange is from 15 Torr to 100 Torr. Accordingly, the electron temperatureis lowered and bombardment of ions is caused even to the anode, wherebythe hard carbon film can be formed even on the anode.

In the coating fabricating apparatus in accordance with the presentinvention, a film which constitutes an object on which a coating hasbeen fabricated is wound on a part of a second cylindrical electrodewhich is grounded and opposite to a first electrode. The rotation of thesecond electrode allows the film to pass between the first and secondelectrodes. A high-frequency voltage is applied to the first electrodewhereby a space between the first and second electrodes produces plasma,and raw gas introduced into plasma is activated, as result of which afilm is fabricated. A peripheral edge portion of the first electrode iscovered with an insulator so that the first and second electrodes andthe insulator constitute a substantially closed space. Gas is suppliedinto the closed space through pores provided in the first electrode.Plasma is shut out within the closed space so that it is difficult toleak plasma to the exterior. An interval between the first and secondelectrodes is 6 mm or less, pressure within the closed space is 15 Torrto 100 Torr.

As mentioned above, in addition to setting pressure within the closedspace to the intermediate pressure, plasma is enclosed in the closedspace, thereby preventing discharge in an undesired region. Further,plasma with higher density is generated so that bombardment of ions tothe anode is increased.

The undesired region is actually directed to a peripheral portion of theelectrode. In the center of the electrode, an electric field has a givenor uniform rate of change. However, in the peripheral portion of theelectrode, in particular, edge portions of the voltage supply electrode,an electric field strength is enlarged, and discharge concentrates inthat region. That is, impedance is lowered with an increase of theplasma density in that region, whereby a large amount of current flowsin that region. Therefore, most of electric power is consumed in theperipheral portion of the electrode so that the plasma density islowered in the center of the electrode. As a result, the electrondensity is increased, and bombardment of ions in the center of theelectrode is lowered.

In the present invention, in order to solve the above-mentioned problem,the peripheral portion of the voltage supply electrode is covered withan insulator in such a manner that plasma is enclosed in the center ofthe electrode.

Further, in the coating fabricating apparatus in accordance with thepresent invention, a film which constitutes an object on which a coatinghas been fabricated is wound on a part of a second cylindrical electrodewhich is grounded and opposite to a first electrode. The rotation of thesecond cylindrical electrode allows the film to pass between the firstand second electrodes. A high-frequency voltage is applied to the firstelectrode in such a manner that a space between the first and secondelectrodes produces plasma. A raw material introduced in plasma isactivated to fabricate a coating. The electrodes are constituted so thatthe strength of an electric field generated on the first and secondelectrodes is strongest on the surface of the first electrode butweakest on the surface of the second electrode. The shortest intervalbetween the first and second electrodes is 6 mm or less, and pressurebetween the first and second electrodes is 15 Torr to 100 Torr.

That is, in addition to setting the pressure to the intermediatepressure, the strength of electric field in the periphery of the firstelectrode (cathode) is heightened in such a manner that the density ofplasma is increased in that region. As the shape of the cathodeelectrode, a knife-shaped or needle-shaped electrode is effective exceptthat edge portions of the plane is used. The ununiform region of thestrength of electric field is positively utilized, thereby to obtainplasma with high density.

Further, in the invention, the first electrode to which a high-frequencyvoltage is applied and the second cylindrical electrode which isgrounded are arranged opposite to each other. Plasma is generatedbetween the first and second electrodes by applying the high-frequencyvoltage thereto. Raw gas introduced in plasma is activated to fabricatea coating. The second electrode is disposed in such a manner that plasmagenerated between the first and second electrodes is sprayed on themetal surface of the second electrode which is grounded. An object(film) on which a coating has been fabricated is wound on a part of thesecond electrode. The rotation of the second electrode allows the filmto pass through a region on which plasma has been sprayed. The electrodeinterval is 6 mm or less, and pressure between the first and secondelectrodes 15 Torr to 100 Torr.

This is a plasma generating apparatus having a structure of a parallelplate or concentric cylindrical electrode, in which plasma with highdensity can be generated by setting the intermediate pressure. Plasma ispositively sprayed onto the substrate by the aid of a gas stream.Because of the intermediate pressure, gas diffusion is delayed incomparison with low pressure, as a result of which there is a case whereradical transport rate is determined. However, this problem is solved byspray of plasma.

Further, in the present invention, gas to be supplied within a plasmaspace consists of mixture gas of hydrogen and gas selected fromhydro-carbon, carbon halide and hydro-carbon halide, or mixture gas ofits mixture gas and rare gas.

A high-speed film formation can be realized by setting the intermediatepressure, however, there arises a problem such as adhesion of the filmonto the cathode. This can be solved by adding carbon halide thereto.

In the present invention, bombardment of ions onto the anode side iseffected thereby being capable of fabricating a hard carbon film even onthe anode side. However, since self-bias applied on the cathode side islarger than that on the anode side, bombardment of ions on the cathodeside is stronger than that on the anode side. In this embodiment, usingthis phenomenon, halogen based gas having etching action is added to rawgas whereby no film formation but etching is conducted on the cathodeside.

Carbon halide, for example, carbon tetrafluoride is known as etchinggas. Carbon tetrafluoride has only etching action, and in dicarbonhexafluoride, tricarbon octafluoride or the like, etching or filmformation is performed according to the strength of self-bias. That is,in the case where self-bias is strong and bombardment of ions is alsostrong, etching is performed. Inversely, in the case where self-bias isweak and bombardment of ions is also weak, the film formation isperformed. In the present invention, bombardment is stronger on thecathode side onto which it is not desired to form a film, which is veryconvenient to the invention.

Accordingly, film fabrication on the cathode side can be restrained andoccurrence of flakes can be restrained. Further, the maintenanceduration for the apparatus can be extended, thereby greatly contributingto an improvement in through-put and a reduction in the costs. In thecase of a super LSI process or the like, it causes contamination,however, in fabricating a carbon film as in the present invention, theredoes not arise a problem such as contamination. When the film whichconstitutes an object is electrically conductive, a film can be disposednot on the cathode side but only on the anode side. The presentinvention is effective even in such a case.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention, andtogether with the description, serve to explain the objects, advantagesand principles of the invention. In the drawings,

FIG. 1 is a diagram showing a structure of a plasma CVD apparatus forfabricating a hard carbon coating in accordance with an embodiment ofthe present invention;

FIG. 2 is a diagram showing a structure of a general film formingapparatus;

FIG. 3 is a schematic diagram showing a structure of a piezoelectricelement in the apparatus shown in FIG. 1;

FIG. 4 is a diagram showing a plasma CVD apparatus in accordance withanother embodiment of the present invention;

FIG. 5 is a diagram showing a structure of an apparatus for forming ahard carbon coating on a tape-shaped material;

FIG. 6 is a diagram showing a structure of a plasma generating apparatusfor generating beam-shaped plasma;

FIG. 7 is a cross-sectional view showing an electrode taken along theline A-A′ in FIG. 6;

FIG. 8 is a cross-sectional view showing another electrode taken alongthe line A-A′ in FIG. 6;

FIG. 9 is a diagram showing a structure of a film forming apparatus inaccordance with an embodiment of the present invention;

FIGS. 10A and 10B are diagrams of an arrangement of the plasmagenerating apparatus, respectively;

FIG. 11 is a diagram showing a structure of the plasma generatingapparatus;

FIG. 12 is a graph showing relationships between pressure and atemperature of electrons and between pressure and a sustaining voltage;

FIG. 13 is a cross-sectional view showing the plasma generatingapparatus;

FIG. 14 is a graph showing relationships between pressure and atemperature of electrons and between pressure and a density ofelectrons; and

FIG. 15 is a cross-sectional view showing another plasma generatingapparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described in more detailwith reference to the accompanying drawings.

First Embodiment

FIG. 1 is a diagram showing a structure of a plasma CVD apparatus forfabricating a hard carbon coating on a surface of formation while theobject 113 is ultrasonic-z,999

The plasma CVD apparatus includes a pair of electrodes 112 and 114 foractivating reactive gas (producing plasma) within a vacuum vessel 111.An electrode 114 is grounded, and an electrode 112 is connected to ahigh-frequency power supply 115, which generates, for example, ahigh-frequency voltage of 13.56 MHz. In the embodiments according to thepresent invention, in stead of the high frequency power supply 115, anelectromagnetic energy suply for supplying electromagnetic energy can bearranged in the apparatus. In this case, DC, AC (50 Hz to 500 KHz) powersupply can be used. Although not shown, a matching unit and the like aredisposed therein, and in case of need, a heating device using a heateror an infrared light lamp is provided.

A piezoelectric element 121 interposed between a pair of electrodes 122and 123 is disposed on the electrode 112, and the object 113 is disposedthereon. The high-frequency voltage is applied from a power supply 125to the piezoelectric element 121 so that the piezoelectric element 121vibrates at a predetermined frequency. A frequency ranging, for example,from 1 KHz to 100 MHz is used.

It is necessary to insulate the electrode 123 from the electrode 112.Further, in case of need, the object 113 is mechanically fixed to theelectrode 122 with solder material, adhesive material as well as anengaging member (an object fixing member) such as a protrusion or hook.

The piezoelectric element 121 is made of crystal liquid, Rochelle salt,lithium niobate, barium titanate, titanate zirconate (PZP), otherorganic piezoelectric material, piezoelectric ceramics or the like.

FIG. 3 is a schematic diagram showing an enlarged portion of thepiezoelectric element 121. There are provided the pair of electrodes 122and 123, the substrate 113 on which a hard carbon coating is formed, andthe high-frequency power supply 12 5 for applying voltage to thepiezoelectric element 121. The object 113 vibrates as the piezoelectricelement 121 is ultrasonic-vibrated.

The vibrating direction and the vibrating number of the piezoelectricelement 121 can be selected in accordance with its cut direction. As thevibrating direction, the thickwise and the face-direction of thepiezoelectric element 121 can be selected. Also, sliding vibration anddeflection vibration can be set as vibrating modes. A vibrating outputis controlled in such a manner that voltage applied to the piezoelectricelement 121 is monitored by a monitor such as a voltmeter or the like(not shown).

Further, in the plasma CVD apparatus shown in FIG. 1, it is preferableto make an interval between the pair of electrodes 112 and 114 as narrowas possible. This is a fact obtained from an experimental result. Whenthe electrode interval is 10 mm or less, a film forming speed isincreased to thereby obtain a high-quality film. However, since theelectrode interval is limited by the thickness of the piezoelectricelement 121 and the thickness of the object 113, in fact, it ispreferable to set the electrode interval to 20 mm or less.

Hereinafter, conditions where the hard carbon coating is formed on theobject 113 will be exhibited. In this case, a silicon substrate is usedas the object 113.

-   -   Raw gas: ethylene gas+hydrogen gas    -   Operating pressure: 80 Pa    -   High-frequency power: 1.5 LW    -   Object temperature: non-heating

The hard carbon coating formed on the object 113 under the aboveconditions has less peeling from the object 113 and has a fine filmquality.

Second Embodiment

In this embodiment, ultrasonic vibrations are given to the object 113 bya method different from that of the first embodiment. FIG. 4 is adiagram showing a structure of a plasma CVD apparatus in accordance withanother embodiment of the present invention. As is apparent from FIG. 4,the electrode 11 2 is vibrated by a ultrasonic vibrator 141 so that theobject 11 3 is ultrasonic-vibrated. Further, with such a structure, theinterval between the pair of electrodes 112 and 114 can be set to 10 mmor less to thereby realize high-speed film formation.

Third Embodiment

FIG. 5 is a diagram showing a structure of a plasma CVD apparatus forforming a hard carbon coating on a tape-shaped material such as amagnetic tape. The plasma CVD apparatus shown in FIG. 5 includes avacuum vessel 111, a roll-shaped electrode 155, an electrode 156 pairedwith the electrode 155, guide rollers 153 and 154 which also act asultrasonic vibrators, a take-out drum 151 or 152, and a wind-up drum 152or 151. Although not shown, a gas introducing system and a gasexhausting system for reactive gas, doping gas as well as dilution gasor the like are also provided in the apparatus.

As the tape-shaped object, a band-shaped object formed of a polyimidefilm is used. The band-shaped film object 157 travels from one drum 151or 152 to the other drum 152 or 151 while it is taken up. Thedrum-shaped electrode 155 is connected to a high-frequency power supply115 of 13.56 MHz so that discharge is generated between the electrode155 and the electrode 156 which is a grounded electrode. At this time,the electrode 155 rotates in accordance with movement of the object 157.As a result, a hard carbon coating is formed on the surface of thefilm-shaped object 157.

That is, the respective drums and rollers are rotated in such a mannerthat the film-shaped substrate travels from one drum to the other drum,and at this time, the hard carbon coating is fabricated on the surfaceof the film-shaped substrate. Hereinafter, one example of an actual filmforming condition will be exhibited. In this case, the substrate travelspeed is 50 m/min, and the thickness of the hard carbon coating is 200Å.

-   -   Object: polyimide film having a width of 180 mm    -   Applied power: 1.5 KW    -   Film forming pressure: 100 Pa    -   Electrode interval: 10 mm    -   Film forming gas: C₂H₆ 200 sccm, H₂ 50 sccm

The film quality of the hard carbon film thus formed is measured inaccordance with Raman spectrum. As a result, it is confirmed that itexhibits a feature of a diamond-shaped carbon film and is a high-qualityhard carbon coating.

With the above-mentioned structure, ultrasonic vibrations can be givento the film-shaped object 157 from the guide rollers 153 and 154 asface-vibrations. After completion of the film formation, the object 157is wound up on the drum 151 or 15 2. A residual of a reactive productwhich does not form a film remaining on the surface of the object 157can be removed by ultrasonic vibrations applied from the guide roller154 or 155.

As a method of giving ultrasonic vibrations onto the object 157, withoutdepending upon the guide rollers 153 and 15 4, a separate ultrasonicvibrator may be arranged in contact with the substrate 157 to giveultrasonic vibrations thereto. The electrode 155 which is in contactwith the object 157 may be vibrated by ultrasonic waves.

In this embodiment, a frequency of 13.56 MHz is used as a high-frequencyfor generating discharge, however, the invention is not limited to or bythis frequency. Also, pulse discharge may be used. As raw gas for thehard carbon coating, hydrocarbon gas such as methane, alcohol or thelike may be used. Further, when forming a film, hydrogen, other additivegas or doping gas may be introduced.

Fourth Embodiment

The structure of an apparatus, a film forming condition and the like inthis embodiment are the same as in the first embodiment. In thisembodiment, a magnetic recording medium such as an optical magnetic disccan be used as the object. In this case, an organic resin plate is usedas the object instead of the photo-electro-magnetic disc. As a result,the hard carbon coating formed on the object has less peeling from theobject and has a fine film quality.

Fifth Embodiment

In this embodiment, a magnetic recording medium such as an opticalmagnetic disc is used as the object, and except for using the magneticrecording medium, the structure of a device, a film forming conditionand the like in this embodiment are the same as in the second embodimentto thereby realize high-speed film forming.

Sixth Embodiment

The structure of an apparatus, a film forming condition and the like inthis embodiment are the same as in the third embodiment. In this case, abelt-shaped object formed of a polyimide film (object material of amagnetic tape) is used as the tape-shaped object.

As a result that the film quality of the hard carbon coating formed ismeasured by Raman spectrum, it is confirmed that it exhibits the featureof the diamond-shaped carbon film and is of a high-quality hard carboncoating. Also, the hard carbon coating has the number of pin-holes of30/mm² or less and is excellent in blocking action against chemicalresistance and moisture.

Ultrasonic vibrations generated by the guide rollers 153 and 154 aregiven to the object 157. As a result, the film-shaped object 157, whichis slender in the form of a tape, can be prevented from contracting in aplace other than a reactive space (a space defined between theelectrodes 155 and 1 56 where discharge is generated). Further,particles attached to the surface of the substrate 157 can be removed tothereby improve the productivity.

Seventh Embodiment

In this embodiment, in the apparatus shown in FIG. 5, described in theseventh embodiment, the drum-shaped electrode 1 55 vibrates. In order toultrasonic-vibrate the electrode 155, the ultrasonic vibrators may bearranged so as to be in contact with the electrode 155.

As indicated by arrows in FIG. 5, there are two cases, that is, onebeing that the electrode 155 is vibrated in a direction of aninter-electrode, that is, in a direction perpendicular to an object 157(up and down direction on the figure) and the other being that theelectrode 155 is vibrated in a direction perpendicular to theinter-electrode direction, that is, in a direction parallel to thesubstrate 157 (right and left direction on the figure).

In the case where the electrode 155 is vertically ultrasonic-vibrated, astate in which AC bias is applied thereto can be realized. In theapparatus as shown in FIG. 5, since the electrode 155 connected to thehigh-frequency power supply 115 is negatively charged with respect tothe grounded electrode 15 6, it comes to a state where negative bias(called “self-bias”) is applied thereto. In this state, when theelectrode 155 is vertically vibrated, then the object 157 is acceleratedso as to approach or go away from ions, which are accelerated in thedirection of the electrodes 155, in a given cycle (a cycle determined inaccordance with the frequency of ultrasonic vibrations applied to theelectrode 155). That is, it comes to a state where AC bias is applied tothe electrode 155.

The above-mentioned state is also obtained by giving ultrasonicvibrations to the guide rollers 153 and 154, however, it is moreremarkable in the case where ultrasonic vibrations are given to theelectrode 155. Accordingly, its effect can be more remarkable in thecase where ultrasonic vibrations are given to a flexible object (forexample, an object such as a magnetic tape).

Eighth Embodiment

In this embodiment, a surface protective film for an optical magneticdisc as a magnetic recording medium. The optical magnetic disc and anoptical disc memory have been widely known as a recording medium such asa CD (compact disc). They are made of organic resin or industrialplastic material and therefore high in productivity and easy inhandling. However, a protective film for protecting a surface layer isnecessary therefor. The protective film is required to transmit light ina visible light region (in general, a semiconductor laser beam of 700 to800 nm) and also to provide high hardness and adhesion.

As the protective film satisfying such demands, there is proposed a hardcarbon coating formed by the apparatus shown in FIG. 1 or 3. An opticaldisc may be used for an object. Since the hard carbon coating can beformed without heating, it is optimum as the surface protective film forthe optical disc using material weak in heat.

In the above-mentioned embodiment, a frequency of 13.56 MHz is used togenerate discharge, however, the invention is not limited to or by thisfrequency. Also, pulse discharge may be used. Further, in addition tohigh-frequency discharge, DC or AC bias may be applied thereto.

As raw gas for the hard carbon coating, hydro-carbon gas such asmethane, alcohol or the like may be used. Furthermore, when forming afilm, hydrogen, other additive gases or doping gas may be introduced.

By fabricating the hard carbon coating while giving ultrasonicvibrations to the object or substrate, a film with fine and excellentqualities can be obtained. In particular, the remarkably fine carboncoating having the number of pin-holes of 30/mm² or less can beobtained, and therefore significantly useful for the protective film.

Ninth Embodiment

In FIG. 9, a polymeric substrate material 3 conveyed from a supply roll2 installed in a vacuum vessel 1 travels through a free roller guide 4and then along a cylindrical can 7 in a direction indicated by an arrow.A polyimide film having a width of 4 cm and a thickness of 6 μm is usedfor the polymeric substrate material 3.

Metal atoms evaporated by an evaporating source 6 are deposited on thepolymeric substrate material 3 to thereby form a magnetic layer having athickness of 0.15 to 0.18 μm. A Co—Cr—Ni alloy is used for theevaporated material. In this case, a piercing type electronic guncapable of scanning over a wide range is used, and the magnetic film isformed at accelerating voltage of 35 KV under operating pressure of5×10⁻⁴ Torr by the electronic beam deposition method. The passing speedof the polymeric substrate material 3 is set to 135 m/min. A shieldingplate 5 is provided for limiting a deposition region.

Potential difference is produced between the cylindrical can 7 and theformed magnetic layer through the free roller guides 4 by a DC powersupply 15. Voltage of 80 V is applied so that the polymeric substratematerial 3 and the cylindrical can 7 are electrostatically in contactwith each other. The polymeric substrate material 3 on which a magneticlayer has been formed is guided to a vacuum vessel 9 through anintermediate roll 8 and then subjected to plasma activating processingtherein. The plasma activating processing will be now described.

Hydrogen gas is introduced from a raw gas supply inlet 18 to a spacebetween a grounded electrode 10 and a high-frequency voltage supplyelectrode 11 which are spaced in parallel to each other at an intervalof 3 cm, and exhausted from an exhaust outlet 19 so that the operatingpressure is controlled to 10⁻¹ Torr to 10⁻² Torr. High-frequency voltageof 13.56 MHz is applied from a high-frequency power supply 12 at a powerdensity of 0.5 W/cm² to thereby generate hydrogen plasma. A plasmaregion 16 is formed so that the polymeric substrate material 3 passesthrough the region 16 in synchronization with a magnetic layer formingprocess.

Through this process, the surface of the magnetic layer is exposed tothe activated hydrogen radical or hydrogen ion. As a result, it isproperly purified to promote the activation of the surface of themagnetic layer. The same effect is also obtained by using argon gas ormixture gas of argon and hydrogen.

It is preferable that a size of a gap (gap through which polymericsubstrate material passes) defined in the partition of buffer chambers20 in the vacuum vessel 9 is smaller than a Debye distance of plasmaproduced in the vacuum vessel 9 or a mean free path under pressure ofthe plasma region 16. This is because plasma is prevented from leakingto the buffer chambers 20.

Subsequently, a vacuum vessel 13 where a coating mainly containingcarbon is fabricated will be described. When the polymeric substratematerial 3 passes through a region where a plurality of beam-type plasmagenerating apparatuses are located, a coating mainly containinghigh-quality carbon is fabricated on the polymeric substrate material 3on which the magnetic layer guided through the free roller guides 4 hasbeen deposited.

FIG. 6 is a diagram showing a structure of a beam-type plasma generatingapparatus for generating beam-type plasma. In the plasma generatingapparatus, using gas mainly containing rare gas such as helium or argon,plasma can be generated under pressure which is larger than 1 Torr andless than 200 Torr, preferably 5 to 150 Torr, more preferably 50 to 100Torr. As rare gas, at least one kind of gas selected from helium, argon,xenon, neon and krypton can be used, or mixture of these rare gases maybe used. Not considering the costs, discharge stability is improved whenusing helium, whereas, considering the costs, argon is advantageous in alow price although it is not excellent in discharge stability incomparison with helium. When using argon, it is desirable that thedielectric constant of an insulator is 9 or more, for example, aluminaor the like.

FIG. 7 shows a cross-section along the lines A-A′ shown in FIG. 6. Asshown in FIG. 7, a coaxial cylindrical electrode is constituted by anouter conductor 29, a center conductor 31 and a cylindrical insulator 33so as to generate discharge. In more detail, discharge is generated in agap 32 between the center conductor 31 and the cylindrical insulator 33.Since gas is not supplied to a gap 34 between the cylindrical insulator33 and the outer conductor 29, no discharge is generated in this gap 34. The mixture gas (gas mainly containing rare gas) of rare gas and rawgas is plasma-processed in the gap 32 when it passes through the gap 32.As a result, beam-shaped plasma is injected toward the exterior of theapparatus so that a film is formed with activated raw gas.

In this embodiment, the center conductor 31 is made of stainless steel,the cylindrical insulator 33 is made of quartz glass, and the outerconductor 29 is made of stainless steel, respectively. It is desirableto use a material having large dielectric constant as the cylindricalinsulator. Also, it is useful to provide irregularities or protrusionson the surface of the center conductor 31 to thereby generate dischargeeasily.

The center conductor 31 is connected to an MHV coaxial connectionclosure 21, and the MHV coaxial connection closure 21 is connected to anAC power supply 12 (refer to FIG. 9) through a coaxial cable (notshown). Electromagnetic energy is supplied between the center conductor31 and the outer conductor 29 by applying AC voltage from the AC powersupply 12. Gas mainly containing rare gas (for example, helium) issupplied from a gas introducing inlet 30 and passes between insulators22 and 27 made of Teflon so as to flow into the gap 32. The Tefloninsulators 22 and 27 serve as members for preventing discharge in aregion where discharge is unnecessary. Casings 23 and 28 are fixed byfastening tools 25 and 26. The casings 23, 28 and the fastening tools25, 26 are made of stainless steel, and have grounded potential likewiseas in the outer conductor 29.

Rare gas of 80% is desirably contained in gas mainly containing raregas. This is because rare gas is mainly plasma-processed under pressureof several Torr and more, raw gas is activated by energy of rare gaswhich has been plasma-processed, and raw gas (for example, methane) isnot almost directly activated. Also, unnecessary gas is exhausted froman exhaust outlet 19 (refer to FIG. 9).

O-rings 24 for sealing are provided between the respective members sothat gas mainly containing introduced rare gas is prevented from leakingfrom gaps between the respective members. A conductive metal foil isfilled up between the cylindrical insulator 33 and the outer conductor29. Therefore, no gas flows into the gap between the cylindricalinsulator 33 and the outer conductor 29. Gas may flow into the gap.

A distance between the polymeric substrate material 3 and the centerconductor 31 is 2 mm, a diameter of the center conductor 31 is 5 mm, anouter diameter of the cylindrical insulator 33 is 22 mm, a thicknessthereof is 1 mm, and a length of the electrodes is 30 arm. When gascontaining helium of 90% is used as rare gas, plasma having a diametermore than 20 mm is generated. As shown in FIGS. 10A and 10B, four plasmagenerating apparatuses 52 are located so that each apparatus generates30 plasma 51. As a result, a coating mainly containing carbon can beuniformly formed on the surface of the polymeric substrate material 3having a width of 40 mm. The four plasma generating apparatuses areinstalled in a unit 41 shown in FIG. 9 in such a manner thathigh-frequency power of 13.56 MHz and 500 W is supplied from the powersupply 12 to the respective plasma generating apparatuses.

The plasma generating apparatus shown in FIG. 6 generates plasma bydischarging between the electrodes constituted in the coaxial form, andthe plasma is injected in the form of beams toward the exterior of theapparatus. As mentioned above, discharge is performed with gas mainlycontaining rare gas. When a coating mainly containing carbon is formed,hydro-carbon gas is added to rare gas as raw gas such as methane oralcohol. Further, in an electrode structure which will be describedlater, raw gas is specially supplied so as to form a film.

In this embodiment, using helium as rare gas and methane as raw gas, acoating mainly containing carbon can be formed. The film formingcondition will be described below.

-   -   Applied power: 500 W    -   Pressure: 100 Torr    -   Gas:Helium:Methane=100 sccm:10 sccm

Plasma is generated by a low-temperature glow discharge, and itstemperature is 100° C. or less. Therefore, an excellent film can beformed even though a substrate is a polymeric substrate material.

Using a bias power supply 42 shown in FIG. 9, DC or AC (high frequencyor the like) bias voltage can be applied to a surface on which thecoating is to be fabricated. Further, magnetic field is produced on thesurface on which the coating is to be fabricated, whereby beam-shapedplasma can be effectively injected on the surface on which the coatingis to be fabricated.

The arrangement of the plasma generating apparatus as shown in FIGS. 10Aand 10B can be freely set in accordance with a required film formingspeed or a size of the surface on which the coating is to be fabricated.For example, if the structure as shown in FIGS. 10A and 10B isadditionally provided, i.e., a pair of structure as shown in FIGS. 10Aand 10B are arranged, a film forming speed can be twice.

The coating mainly containing carbon is formed at a passing speedcooperating with the above-mentioned two processes, thereby obtaining afilm thickness of about 200 Å. The polymeric substrate material 3 onwhich the film has been formed is restored to a winding-up roll 14through the free roller guide 4.

In the embodiment of the present invention, in processes before themagnetic layer is formed, irradiation of ions, electrons or the like,orheating or the like can be performed using a known technique if need.Also, in this embodiment, a polyimide film is used as the object,however, metal resin, plastic or the like may be constituted in the formof a roll or plate.

The magnetic recording medium fabricated in this embodiment is cut intoa tape having a width of 8 mm and evaluated in reproduced output anddurability by using a 8-mm video deck on the market. As a result, when athickness of the coating mainly containing carbon is 200 Å or more, astable reproduced output excellent in running stability and stilldurability and reduced in drop-up is obtained. Further, apart from thenormal reproducing operation, even in the continuous and discontinuoustest for a special reproducing operation, it is confirmed that excellentdurability is exhibited.

Tenth Embodiment

In this embodiment, in the structure shown in ninth embodiment, reactionproducts are prevented from adhering to a coaxial discharge electrodeportion. A difference in structure between this embodiment and the ninthembodiment is that a hollow portion 30 is provided in the centerconductor (center electrode) 31 in such a manner that raw gas flows intothe hollow portion 30, as shown in FIG. 8. FIG. 8 shows anothercross-section along the line A-A′ as shown in FIG. 6.

In such a structure, rare gas (for example, helium) flows into the gap32, and raw gas (for example, methane) flows into the hollow portion 30.Since discharge is not generated in the hollow portion 30, raw gas isnot entirely activated and exhausted to the exterior of the apparatus.On the other hand, rare gas flowing into the gap 32 is plasma-processedby high-frequency discharge generated between the center conductor 31and the outer conductor 29. The rare gas, which is not activated in theexterior of the apparatus, is coaxially enclosed with theplasma-processed rare gas, and activated or plasma-processed by plasmaenergy of rare gas.

Since raw gas is activated in the exterior of the apparatus, thepossibility that reaction products adhere inside the apparatus andflakes produces can be fundamentally removed. Also, in the exterior ofthe apparatus, since raw gas is enclosed with the plasma-processed raregas, its collecting efficiency can be significantly heightened.

Eleventh Embodiment

In this embodiment, a sheet-shaped (plate-shaped) plasma generatingapparatus is used as the plasma generating apparatuses disposed in theunit 41 as shown in FIG. 9. The structure of this sheet-shaped plasmagenerating apparatus is shown in FIG. 11. The apparatus has a parallelplate electrode. Plasma generated by the parallel plate electrode isdischarged to the exterior of the apparatus as plate-shaped plasma.

In FIG. 11, a parallel plate electrode includes an electrode plate 61,an insulator plate 63 and an outer casing 6 2. The insulator plate 63 isprovided in close contact with the outer casing 62. In this embodiment,the electrode plate 61 is made of stainless steel, the insulator plate63 is made of quartz glass, and the outer casing 62 is made of stainlesssteel. The electrode plate 61 is insulated from other members withTeflon shields 620, 621 and 622 and connected to the MHV coaxialconnection closure 611. An AC power supply 64 (corresponding to the biaspower supply 42) having a frequency of 13.56 MHz supplies AC voltage tothe electrode plate 61 through a coaxial cable (not shown.) connected tothe MHV coaxial connection closure 611.

Rare gas supplied to a space between the electrode plate 6 1 and theinsulator plate 63 is introduced from a gas introducing inlet 612 andpasses through a gas groove carved on an insulator 613 made of Teflon.The Teflon insulator 613 also serves as a member for preventingdischarge on a place where no discharge is necessary. The outer casing62 and an electrode plate holder 616 are fixed on a roof 617. Theelectrode plate holder 616 and the roof 617 are made of stainless steel,and they as well as the outer casing 62 have grounded potential. A widthof the insulator plate 63, that is, a discharge portion width (in FIG.11, a length of the electrode in the depth direction) is 25 mm, and athickness of the insulator plate is 1.0 mm. Also, an electrode intervalis 5 mm, an electrode length (in FIG. 11, a length in the longitudinaldirection) is 3 0 mm. Therefore, sheet-type plasma of about 5 mm×25 mmis generated.

Helium of 100 sccm is supplied to the above-mentioned apparatus, andhigh-frequency power of 500 W (frequency of 13.56 MHz) is suppliedthereto under pressure of 100 Torr. In this case, stable discharge isobtained in a discharge region so that sheet-shaped plasma can bedischarged to the exterior of the aparatus. Even though this state ismaintained for ten minutes or more, no trouble such as overheating iscaused.

As a result of measuring a temperature of plasma formed by dischargingwith a thermocouple, a temperature of approximately a room temperatureto 70° C. is obtained, and therefore it is confirmed thatlow-temperature glow discharge is generated.

When this embodiment is applied to the structure shown in FIG. 9, it ispreferable to use raw gas such as methane or ethylene as additive gas.Also, when a film forming speed is increased or a film forming area isenlarged, a plurality of plasma generating apparatuses may be arrangedas shown in FIGS. 10A and 10B.

Twelfth Embodiment

In this embodiment, the unit 41 shown in FIG. 9 is constituted as shownin FIG. 13. In FIG. 13, as the film-shaped object (substrate) 81, forexample, a magnetic recording medium tape is used. A cathode electrode82 is supported by an insulator 83 and connected to a high-frequencypower supply 87 through a matching box 86. A cylindrical electrode 85used as an anode electrode is grounded. High-frequency discharge isgenerated in a plasma reaction space 89 between the anode electrode 85and the cathode electrode 82. The anode electrode 85 can be rotatedwhereby the film-shaped object 81 is smoothly moved. Raw gas, rare gas,and additive gas are guided into the plasma discharge space 89 through agas introducing pipe 84. These gases are guided from the gas introducingpipe 84 to fine holes 88 provided in the anode electrode 82 and injectedinto the plasma reaction space.

A width of-the cathode electrode 82 (a width effective in discharge) is20 mm, and a length thereof is 30 cm. Also, a diameter of thecylindrical anode electrode 85 is 20 mm, and a length thereof is 30 mm.An interval between the cathode electrode 82 and the anode electrode 85is 5 mm. An interval of the paired electrodes is desirably 10 mm orless.

An organic resin film having a width of 10 inches on which a metalmagnetic substance has been deposited is used as a film-shaped object81, and a hard carbon coating having a thickness of 300 Å is formed onthe surface of that film. The film-shaped object travels at 12 m/min (20cm/sec). In this case, the film-shaped object travels in the dischargespace 89 having a width of 20 mm for 0.1 seconds. Therefore, in order toform a film having a thickness of 300 Å, a film forming speed of 3000Å/sec is necessary. Hereinafter, the conditions under which the filmforming speed of 3000 Å/sec is obtained will be exhibited.

-   -   Reaction pressure: 60 Torr    -   Applied power: 300 W (5 W/cm²) (13.56 MHz)    -   Raw gas: C₂H₄:H₂:Ar=1:1:2 (1000 sccm in total)    -   Additive gas: C₂F₆ (addition of 10% to C₂H₄)

The film forming speed of 3000 Å/sec can be obtained under theabove-mentioned conditions, whereby the hard carbon film having athickness of 300 Å can be formed on the surface of the film-shapedobject 81.

C₂F₆ is used as additive gas for the following reason. That is, ingeneral, when the film is formed at the above-mentioned speed, a largeamount of reaction products adhere to the cathode electrode. Thereaction products come to flakes, resulting in an obstacle to the filmformation. Therefore, an arrangement in which no reaction productsadhere to the cathode electrode is required.

On the other hand, in the structure shown in FIG. 13, the cathodeelectrode 82 is biased at minus potential by action of self-bias, andplus ions produced by plasma discharge is attracted to the cathodeelectrode 82 side. As a result, the cathode electrode 82 side isspattered.

When C₂F₆ is used as additive gas as in this embodiment, since thecathode electrode 82 side is spattered and etched, the reaction productsadhering to the cathode electrode 82 are etched simultaneously whenadhering. Therefore, the hard carbon film can be formed without adheringthe reaction products to the cathode electrode.

C₂F₆ is used because C₂F₆ has etching action of F and hard carbon filmforming action of C. CF₄ can be used as additive gas. However, since CF₄has no hard carbon film forming action, it is desirable to use C₂F₆which contributes to the hard carbon film formation.

Thirteenth Embodiment

FIG. 15 is a diagram showing an apparatus for fabricating a coating onthe surface of a tape- or film-shaped object. In this embodiment, inparticular, a case where the carbon coating is fabricated on the surfaceof the magnetic tape as the surface protective film will be described.

In the film-shaped object 81, a film made of magnetic material is formedon the surface of resin material such as polyimide or the like by thevapor deposition method, and the like. The cathode electrode 82 isconnected to the high-frequency power supply 87 through the matching box86. A can roll 801 constituting an anode electrode rotates during filmformation in order to convey the substrate 81. The electrode 80 1 isconstituted so that it is ultrasonic-vibrated by piezoelectric elements.That is, ultrasonic-vibrations are given to the object 81 by thecylindrical electrode 801 during film formation so that the object 81 isultrasonic-vibrated.

In the film formation, high-frequency discharge is generated in theplasma reaction space 89 between the anode electrode 801 and the cathodeelectrode 82. From a gas introducing pipe 84, methane of raw gas anddilution gas such as hydrogen are introduced into the plasma reactionspace 89. The gases blow off from the fine holes 88 provided in thecathode electrode 82 into the reaction space 89.

The width of the cathode electrode is set to, for example, 20 mm, and alength thereof is set to, for example, 30 cm. A diameter of thecylindrical anode electrode 801 is set to, for example, 20 mm, a lengththereof is set to, for example, 30 cm or less. In this case, thesubstrate 81 having a width of 30 cm or less can be used.

An example of the film forming conditions in the case where theelectrode having the above-mentioned dimensions is used.

-   -   Reaction pressure: 60 Torr    -   Applied power: 300 W (13.56 MHz)    -   Raw gas: C₂H₄:H₂:Ar=1:1:2 (1000 sccm in total)    -   Additive gas: C₂F₆ (addition of 10% to C₂H₄)    -   Ultrasonic frequency: 30 KHz (Ultrasonic vibrations given to        electrode 801)

The ultrasonic vibrations are given to the object 81 through theelectrode 801, whereby restraining the flakes of the reaction productsfrom adhering to the surface of the substrate 81 on which a coating isto be fabricated. As a result, a fine carbon coating without pin-holescan be fabricated.

As described above, by etching the cathode electrode with halogen gas,the film can be formed on the surface of the film-shaped object on theanode electrode side without adhering the reaction products to thecathode electrode.

The present invention can provide an apparatus for fabricating a hardcarbon film with stability and high reliability on the surface of themagnetic recording medium having the conductive metal magnetic layer.

Further, although a carbon film with sufficiently high hardness couldnot be fabricated on the grounded electrode side in the conventionalapparatus, the present invention can provide an apparatus capable offabricating the carbon film having sufficient wear resistance andlubricity even in a state where it is in contact with the anode which isa grounded electrode.

Furthermore, the present invention can provide an apparatus for forminga film at a high speed capable of fabricating the hard carbon film as aprotective film at the same time of the magnetic layer fabricatingprocess.

Further, the present invention can provide an apparatus for restrainingoccurrence of flakes caused by dirts on the electrodes produced by thehigh-speed film formation. As a result, the film formation on thecathode side can be restrained so that occurrence of the flakes can berestrained. Moreover, since the maintenance period of the apparatus canbe extended, through-put can be improved, thereby greatly contributingto reduction in the costs.

Further, the magnetic recording medium fabricated by the fabricatingapparatus of the present invention has a high quality which is improvedin adhesion and surface characteristics between the medium and thecoating mainly containing the magnetic layer and carbon. Furthermore,although a low-grade oxide fabricated on the surface of the magneticlayer may not be fundamentally removed only by preventing it from beingexposed from the atmosphere, in that case, the plasma activating processin accordance with the present invention is effective in removal of theoxide. Further, the surface characteristics of the coating mainlycontaining carbon, that is, wear resistance, high smoothing property,hardness and the like are remarkably improved, thereby enabling themanufacture of the magnetic recording medium which is sufficientlyworthy in industry, and the rate-determining point on continuousfabrication which was a problem in the conventional apparatus can bealso prevented.

The foregoing description of preferred embodiments of the invention hasbeen presented for the purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed, and modifications and variations are possible in lightof the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and its practical application to enableone skilled in the art to utilize the invention in various embodimentsand with various modifications as are suited to the particular usecontemplated. It is intended that the scope of the invention be definedby the claims appended hereto, and their equivalents.

1. A method for forming a film comprising the steps of: preparing afilm-formation apparatus having a first electrode and a secondelectrode; generating a plasma between the first electrode and thesecond electrode by applying an electromagnetic energy which is combinedwith plural frequencies to the first electrode; and activating amaterial gas by the plasma.
 2. A method according to claim 1, wherein agap between the first electrode and the second electrode is 6 mm orless.
 3. A method according to claim 1, wherein a rare gas is added tothe material gas.
 4. A method according to claim 3, wherein the rare gasis at least one of helium and argon.
 5. A method for manufacturing afilm comprising the steps of: preparing a film-formation apparatushaving a first electrode and a second electrode; generating a plasmabetween the first electrode and the second electrode by applying anelectromagnetic energy in which a low frequency and a high frequency arecombined to the first electrode; and activating a material gas by theplasma.
 6. A method according to claim 5, wherein a gap between thefirst electrode and the second electrode is 6 mm or less.
 7. A methodaccording to claim 5, wherein a rare gas is added to the material gas.8. A method according to claim 7, wherein the rare gas is at least oneof helium and argon.
 9. A method for manufacturing a film comprising thesteps of: preparing a film-formation apparatus having a first electrodeand a second electrode; generating a plasma between the first electrodeand the second electrode by applying an electromagnetic energy in whicha low frequency of 1 KHz to 1 MHz and a high frequency of 10 to 100 MHzare combined to the first electrode; and activating a material gas bythe plasma.
 10. A method according to claim 9, wherein a gap between thefirst electrode and the second electrode is 6 mm or less.
 11. A methodaccording to claim 9, wherein a rare gas is added to the material gas.12. A method according to claim 11, wherein the rare gas is at least oneof helium and argon.
 13. A method for manufacturing a film comprisingthe steps of: preparing a film-formation apparatus having a firstelectrode and a second electrode; generating a plasma between the firstelectrode and the second electrode by applying a frequency of severaltens KHz to several GHz to the first electrode; and activating amaterial gas by the plasma.
 14. A method according to claim 13, whereina gap between the first electrode and the second electrode is 6 mm orless.
 15. A method according to claim 13, wherein a rare gas is added tothe material gas.
 16. A method according to claim 15, wherein the raregas is at least one of helium and argon.